The present invention relates to a tool for allowing an abrasive grit material to be incorporated into a coating being applied to a tip of at least one workpiece, such as an airfoil blade, in an array of workpieces, such as an array of airfoil blades used in jet engines, and to a method for repairing at least one airfoil blade in an array of airfoil blades using said tool.
It is known to provide at the tip of a gas turbine blade a tip portion comprising abrasive particles which are embedded in a matrix, the tip being intended to run against the surface of a shroud of a material which is softer than the abrasive particles. By this means, it is possible to produce, by the abrasive action of the particles on the shroud, a gap between the tip and the shroud which is very small, thus minimizing gas losses. U.S. Pat. Nos. 4,169,020, 4,232,995 and 4,227,703, all to Stalker et al., illustrate a turbine blade tip having a coating containing abrasive particles entrapped within a metal matrix. The abrasive tip portion is formed by depositing abrasive particles and a metal matrix concurrently on an inner tip portion of the turbine blade after the inner portion has been bonded to a projection body. This codeposition of matrix material and particles is accomplished electrolytically from an electrodeposition bath in which there are suspended abrasive particles formed from aluminum oxide, cubic boron nitride (CBN), or other abrasive carbides, oxides, silicides, or nitrides.
U.S. Pat. No. 4,608,128 to Farmer et al. illustrates a method for applying abrasive particles to an article surface which includes providing an electrically non-conductive tape and particle member for use in an electrodeposition type system. The tape includes pores large enough to allow passage of electrodeposition current and electrolyte solution but smaller than the size of the abrasive particles to be retained on the tape. The tape has a porous adhesive layer of relatively low tack level, the adhesive carrying the abrasive particles through a first or relatively weak bond. A metallic coating is electrodeposited through the pores of the tape and the adhesive onto the article surface and about the abrasive particles in contact with the surface. This bonds the abrasive particles to the article surface primarily through a second bond between the metallic coating and the abrasive particle which is stronger than the first, relatively weak bond. Thereafter, the tape and particle member is separated at the first bond from the abrasive particles bonded to the article surface.
U. S. Pat. No. 4,610,698 to Eaton et al. illustrates a process wherein a combination of sintering, plasma arc spraying, hot isostatic pressing and chemical milling is used to form an abrasive surface on a turbine blade. Alumina coated silicon carbide particulates are clad with nickel and sinter bonded to the surface of a superalloy turbine blade tip. An impermeable layer of plasma arc sprayed superalloy matrix is deposited over the particulates and then has its inherent voids eliminated by hot isostatic pressing. The abrasive material so formed on the surface is then machined to expose the particulates. Next a portion of the matrix is removed so that the machine particulates projected into space and are thus best enabled to interact with the abradable ceramic air seals in a gas turbine engine.
U.S. Pat. Nos. 4,818,833 to Formanack et al. and 4,851,188 to Schaefer et al. illustrate yet another method and apparatus for fabricating a turbine blade having a wear resistant layer sintered to the blade tip surface. The abrasive, wear resistant layer is applied to the tip surface of a superalloy gas turbine blade by a high temperature sintering operation which reduces a high strength bond between the layer and the blade, minimizes gamma prime phase growth, and prevents recrystallization in the blade. An inductively heated graphite susceptor is used to heat the blade and a refractory metal shield is used to surround the airfoil and root portions of the blade while leaving the tip portion exposed to the heat source.
U.S. Pat. No. 4,884,820 to Jackson et al. relates to a wear resistant, abrasive laser-engraved ceramic or metallic carbide surface for rotary labyrinth seal members. The tip is provided with a ceramic or metallic coating bonded thereto. The surface of the coating has a plurality of laser-formed depressions and is used to provide a wear resistant, cutting surface capable of cutting into a second member.
U.S. Pat. No. 5,074,970 to Routsis et al. relates to a method for applying an abrasive layer to titanium alloy compressor airfoils. The method described in this patent includes the application of several layers of nickel, one of which includes abrasive particulates. More specifically, the method comprises the steps of applying a first nickel layer having a thickness of about 12 to 18 microns directly to the blade tip surface; applying a second nickel layer to the first nickel layer, the second layer being less than about 1 micron in thickness; electroplating a third nickel layer onto the second nickel layer, and while the third layer is being electroplated, submerging the blade tip in a slurry of plating solution and electrically nonconductive abrasive particulates disposed upon a membrane permeable to electric current and plating solution, wherein the particulates in the slurry are entrapped in the third layer by the continued electroplating of nickel; applying a fourth nickel layer onto the third nickel layer wherein the combined thickness of the third and fourth nickel layers is between about 50 and 95% of the average particulate dimension; and heat treating the plated component.
U.S. Patent No. 5,076,897 to Wride et al. also relates to a method of producing a gas turbine blade having an abrasive tip. The method described in this patent comprises producing a binding coat on the tip of the blade body by electrodeposition, the binding coat comprising MCrAlY where M is one or more of iron, nickel and cobalt, anchoring coarse particles of an abrasive material to the binding coat by composite electrodeposition of the particles and an anchoring coat from a bath of plating solution having the abrasive particles suspended therein, and then plating an infill around the abrasive particles. The anchoring coat may be of cobalt, nickel or MCrAlY and preferably has a thickness less than 30 microns. The infill material may also be MCrAlY. Preferably, the deposition of the infill is accompanied by vibration of the blade in a direction which is substantially vertical and substantially along the axis of the blade.
In copending patent application Ser. No. 08/138,530, filed Oct. 15, 1993, entitled METHOD AND APPARATUS FOR CBN TIPPING OF INTEGRALLY BLADED ROTORS (Attorney Docket No. EH-9640 m), a method and apparatus for forming an abrasive surface on the tip portions of a plurality of airfoils is described. The apparatus includes a device for containing the abrasive grit material to be incorporated into the coating applied to the tips. The grit containment device described therein has a solid, substantially L-shaped barrier member which is placed over the tip portions of the airfoil blades being coated and a screen attached thereto which surrounds the periphery of the airfoil blades. The screen together with the barrier member define a channel in which the abrasive grit material is located during the coating operation.
Since the development of this containment device, it has been found that the use of the screen in this manner is disadvantageous in that it has insufficient structural strength to withstand bending and other forces which are generated during the coating operation. As a result, grit material can leak into the plating solution and become unretrievable. This is undesirable because the abrasive grit material is quite expensive. Still further, this containment device is not particularly well suited for use in coating a single workpiece.